1. Field of the Invention
The present invention relates to a process for recovering copper from ore, which contains chalcopyrite as a main copper mineral, efficiently with low cost at a mine site.
2. Description of the Prior Art
Copper smelting normally treats a copper concentrate containing copper grade of 20 to 35 wt % and obtained by physical separation process like flotation or gravity concentration from the copper ore, which comprises several copper minerals such as chalcopyrite, chalcocite or bornite or the like.
Pyromotallurgical process is the primary method used to treat copper concentrate, but in the case of uneconomical condition to apply the physical separation process for low grade sulfide ore or oxide ore, hydrometallurgical process is used to produce copper from the ore directly.
Pyrometallurgical process consist of two unit process, treating copper concentrate to produce matte, and a copper conversion process for obtaining blister copper from the matte. The process for producing matte is a process wherein by heating and melting the copper concentrate and oxidizing it by oxygen gas, the iron content in the copper concentrate is oxidized preferentially, producing a slag containing iron and silica as the main constituents, thereby removing the iron, while at the same time producing matte comprising Cu2S. In the conversion process, the produced matte is further oxidized by oxygen gas, and the sulfur which is bonded to the copper is removed as SO2, thereby obtaining blister copper. The blister copper obtained in this manner is then refined to electrolytic copper in an electrolytic refining process.
Pyrometallurgy is economical because blister copper is produced by promoting a spontaneous reaction using the oxidation energy of the sulfides of copper and iron. However, because the sulfur bonded to the copper or iron in the copper concentrate becomes SO2 gas, it is generally recovered and rendered harmless in the form of sulfuric acid, although an environmental problem exists in that harmful SO2 gas which cannot be recovered is released into the atmosphere.
Therefore recently, attention has been focused on wet processes which are not accompanied by the production of SO2 gas. One such process is a hydrometallurgical process known as the SX-EW method, wherein copper can be recovered at low cost by leaching copper from low grade oxide ore or sulfide ore from which copper concentrate cannot be recovered economically, and subsequently using solvent extraction or electrowinning techniques.
In the case of low grade oxide ore, the majority of the leaching reaction is simple acid dissolution. In contrast, in the case of low grade sulfide ore, a solution containing sulfuric acid or ferric ions or the like is poured over an accumulated heap of copper ore, to leach the copper. In other words, sulfuric acid or ferric ions or the like are required for this leaching reaction, and managing the concentrations of the sulfuric acid or ferric ions appropriately is important in terms of the operation of each plant.
The reaction equations for the copper leaching reaction can be expressed as shown in equations 1 through 4, and it is apparent that ferric ions play a large role in the leaching of the copper.CuFeS2+2Fe2(SO4)3=CuSO4+5FeSO4+2S  Equation 1CuFeS2+2Fe2(SO4)3+2H2O+3O2=CuSO4+5FeSO4+2H2SO4  Equation 2 Cu2S+H2SO4+1/2O2=CuSO4+CuS+H2O  Equation 3Cu2S+Fe2(SO4)3=CuSO4+CuS+2FeSO4  Equation 4
The ferrous ions produced by these reactions are oxidized in air to reform ferric ions. These ferric ions are then reused in the leaching of copper.
In addition, if bacteria is involved in the copper ore within the accumulated heap, then sulfuric acid is produced by the bioactivity of the bacteria, and the oxidation of iron ions is accelerated, yielding a rapid improvement in the leaching kinetics. Accordingly, ideas for enhancing the oxidation to ferric ions and the activity of the bacteria, either by improving the way in which the copper ore accumulates, the shape of the heap or the watering method so that air can enter as easily as possible, or by using a blower to introduce air into the heap using a pipe, are currently being trialed.
However, the iron ions in the pregnant leach solution (PLS), which is leached from the copper ore and reused, becomes a hydroxide or a compound with another metal and becomes bound, and as such it is difficult to increase the ferric ion concentration above a certain concentration. As a result, a PLS with the type of high iron ion concentration ideal for leaching cannot be obtained, and to date a high copper leach rate has been unobtainable by heap leaching.
Furthermore, a compound containing ferric ions could be dissolved in the PLS used in the heap leaching in order to improve the copper leach rate, but because the use of reagents which increase the ferric ion concentration incur an increase in costs, this method is not applicable in actual practice.
In addition, the SX-EW method is conventionally used as a method effective in recovering copper from low grade copper ore which was otherwise difficult to process economically, but a long period of time is required to leach the copper. For example, generally one year is required to leach 50% of copper in the case of chalcocite, and approximately 5 years are required to leach more than 70%. Even more time is required for leaching in the case of chalcopyrite, which has poor solubility.
Due to the problems described above, the SX-EW method is normally applied to oxide ore or chalcocite ore. Accordingly, in a copper mine with chalcopyrite as the main mineral, concentrating of the economically viable high grade copper ore is performed to produce a copper concentrate, and a pyrometallurgical process is then performed at various smelteries, and the low grade copper ore considered to be economically non-viable has conventionally been left to accumulate as waste. From an economic point of view, on site smelting is preferred, predominantly due to the copper concentrate smelting costs and the costs of transporting the concentrate, but the use of a pyrometallurgical method near the copper mine is difficult to realize because of the large initial investment required, and because of environmental problems such as the production of SO2 gas.
Accordingly, wet processes have been proposed in which on site smelting can be performed easily, and in which copper is leached by promoting an active chemical reaction. For example, a wet process has been proposed in which sulfide ore or copper concentrate or the like is leached at high temperature and high pressure to recover copper. In this wet process, an autoclave is used to leach copper concentrate at high temperatures, between 100° C. and 230° C. The leaching reaction is as shown in equations 5 and 6, and the copper concentrate is dissolved under pressurized oxygen or air (at an oxygen partial pressure of 0.1 to 2.0 MPa), allowing the copper leach rate to reach 90 to 99%. Because leaching is performed under conditions of high temperature and high pressure, the reaction is completed within 0.5 to 6 hours.2CuFeS2+17/2O2+H2SO4=2CuSO4+Fe2(SO4)3+H2O  Equation 5 2CuFeS2+5/2O2+5H2SO4=2CuSO4+Fe2(SO4)3+4S+5H2O  Equation 6
By careful control, it is possible to leach copper or iron or the like via the reaction in equation 6 without oxidizing the sulfur in the copper concentrate, but in practice, attempting to increase the copper leach rate results in the oxidation of 10 to 95% of the sulfur, and the reaction in equation 5 is considered to be predominant.
Accordingly, in the wet process, the sulfur within the copper concentrate is oxidized to sulfate ions, and sulfuric acid is produced. Consequently, the leachate cannot be processed as-is, and must be neutralized by a neutralizing agent such as limestone or sodium hydroxide before the copper recovery and refining processes are preformed. If the leachate is not neutralized, then when copper is recovered by solvent extraction, a large amount of copper remains in the solution, leading to a reduction in the copper recovery rate. In order to reduce the amount of neutralizing agent which is required, a process has been developed in which the oxidation of sulfur during leaching is kept to a minimum, and the sulfur is recovered in elemental sulfur form, but in this process, leaching must be performed under special conditions, such as the addition of chlorine ions during leaching, which is disadvantageous in terms of the corrosion resistance of the high temperature high pressure vessel based around an autoclave. In this manner, in order to recover copper economically from copper concentrate using a wet process, further improvement is necessary.
A process has also been proposed in which the copper leachate containing the sulfuric acid obtained in the wet process is mixed with the liquid used in the heap leaching process for low grade copper ore, thereby reducing the sulfuric acid concentration of the copper leachate, and the resulting product is then sent to the solvent extraction and electrowinning processes, which function as the processes for recovering copper in heap leaching, thereby obtaining copper (U.S. Pat. No. 5,698,170). This process is gathering attention as a new method for solving the above problems, in which copper can be recovered using existing plant facilities by simply diluting the copper leachate, without neutralizing the sulfuric acid in the copper leachate.
However, because the most suitable sulfuric acid concentration for copper recovery by means of solvent extraction is low, at 0.5 to 10 g/L, attempting to reduce the sulfuric acid concentration of a solution containing a high level of acid, such as the copper leachate, to a concentration within this range by dilution alone, means the mixing ratio of the solution to the heap leaching process solution is restricted. Furthermore, in plants which only produce a small amount of heap leaching process solution, it is impossible to leach a large amount of copper concentrate.
Furthermore, the only advantage of this method is that the sulfuric acid in the copper leachate can fulfill a supplementary role in the heap leaching process of copper ore, and this method requires lengthy leaching time and does not actively improve the leaching reaction in a heap leaching process with a low copper leach rate, and consequently does not have significant advantages.